code: def test_str_lower():
      """Test string conversion to lowercase using `.str.lower()`."""
  
      df = pd.DataFrame(
          {
              "codes": range(1, 7),
              "names": [
                  "Graham Chapman",
                  "John Cleese",
                  "Terry Gilliam",
                  "Eric Idle",
                  "Terry Jones",
                  "Michael Palin",
              ],
          }
      )
  
      expected = df.assign(names=df["names"].str.lower())
  
      result = df.process_text(
          column_name="names", string_function="lower"
      )
  
      assert_frame_equal(result, expected)
  
imports: import numpy
  import pandas
  import pytest
  from pandas.testing import assert_frame_equal
label_desc: Use image node_id 17 for calling a global function with example usage: test_str_lower() without return types